A number of flow measurement and detection arrangements have been developed over the years. These devices may be generally categorized as one-, two- and three-element devices. Single sensor devices can calculate fluid velocity, but are generally not suitable for determining fluid flow direction.
Two- and three-element devices generally operate on the principle of adding heat to a flowing fluid and measuring the heat transfer functions of sensors placed along the fluid flow stream. The difference between the heat transfer functions of the upstream and downstream sensors is used to calculate flow direction and velocity.
For example, U.S. Pat. No. 4,787,251 reports a two-element configuration. The configuration in U.S. Pat. No. 4,787,251 requires that both sensors produce a significant thermal wake. To produce these thermal wakes, both sensors are typically 30.degree.-100.degree. C. above that of the surrounding fluid temperature. The thermal wake is sensed by the downstream sensor. Use of both sensors as heating elements results in unnecessary power loss, in that the heat added by the downstream element is not utilized.
U.S. Pat. No. 4,982,605 describes a two-element configuration for detecting fluid flow velocity in a fume hood. This two-element configuration is utilized to detect fluid flow velocity and provides for temperature compensation due to changes in the ambient fluid flow temperature. This configuration, however, can not detect fluid flow direction which is critical in many applications.
Thus, there exists a need for an inexpensive and reliable fluid flow direction and velocity monitor, including a temperature compensation means, for use in laboratories, hospitals and other ventilation applications where airborne contaminants must be isolated within or outside of a controlled space. Reliable fluid flow direction monitors may be used to maintain minimal pressure differentials between the controlled and non-controlled spaces. Reliable detection of minimal pressure differentials allows for the use of lower volume air supply systems, therefore, reducing energy requirements.